When I was 15 years old I did a 2 week work experience program with Eastern Australian Airlines. They are a regional subsidiary of Qantas operating a fleet of Dash-8 and Dash-3 aircraft from SYD to various regional towns in the state.
I spent several days doing filing and getting people coffee. I watched them write paper tickets, remember this is several years back... I also spent a couple of days at check in, just watching, and chatting to the girls.
But almost everyday, I flew!!!
It was great. In two weeks, I went to Newcastle, Lord Howe Island, Dubbo, Wagga, Port Macquarie and Coffs Harbour!! On three occaisions I was able to sit in the jump seat in the cockpit with a headset, and listen to ATC and other planes. I watched in amazement as the pilot were able to bring the a/c in for landing in what as far as I was concerned ZERO visability!!! I almost SH
T my pants.
On other flights I sat in the last row and watched the F/A. Prior to the meal service, I helped her/him load the drinks on the trolly. During boarding I helped a few people find their seats and stow their bags.
This all gave me a true understanding of what I wanted to do.
In light of recent events in the world, I seriously doubt that the flying thing would still be possible, however most airlines are still happy to offer work experience programs, expecially the small airlines. It is a matter of writing and calling as many as you can, and simply explaining what you want to do when you are older. You dont get paid.
After you have done a little more research into the job itself, if you want to become cabin crew, you need to start getting experience in the customer care industry and go from there.
If you decide to become a pilot, then most likely the RAF or Navy is the way to go. You are provided with excellent training, and after your contract (up to 15 years), I think you can pretty much walk into a high paying job with some of the worlds leading airlines.
If your family has very deep pockets, you can start taking some basic flying lessons, and apply to a cadetship with someone like BA. I think they still do them. You generally need very good High School results, and often be a university graduate, with majors in Maths and Physics. Anyway, I am sure there is heaps more info in the pilots section of this site.
Follow your heart and your dreams, but use your brain...